Such gels are useful for being converted to glass articles by conventional techniques, e.g. mother preforms for the making of optical fibers. Within the present gel matrix, defined hereinafter as a "wet gel", is intimately entrapped a liquid phase containing lower alkanols, water-soluble organic solvents, acid or basic catalysts providing pH's in the approximate range of 4 to 8 and, possibly, dissolved uncondensed silicon and/or other metals compounds such as lower alkoxides.
A randomly taken element of the present gel can be illustrated by the following general formula ##STR1##
in which the symbol ME refers to one or more of the above mentioned doping metals, the role of residual valencies on such metals, if any, being explained later, and R means alkyl groups. In the above formula, the tridimensional branching of the illustrated polysiloxane network results from the polycondensation of neighbouring silanol functions and this is shown in the above representation as a purely arbitrary pattern. However, the average length of the straight chains and the number of cross-links per unit of volume of the gel obviously correlates with the physical properties of the gel like, viscosity, stiffness, module of elasticity, modules of rupture and the like. In the present invention, the gels which are obtained are generally weak solids with low modules of rupture, e.g., in the range of 10.sup.-2 MN m.sup.-2.
In the present gel, the distribution of the substituents (R=lower alkyl groups, ME=metals) and OH depends on the following considerations. Since the gel skeleton originates from the progressive hydrolysis of lower alkoxy silanes and polycondensation of the silanol functions provided by said hydrolysis, the relative number of free OH substituents will depend on the respective rates of hydrolysis of the alkoxides groups and that of the dehydration-condensation of said OH groups. Now, the hydrolysis and polycondensation is homogeneous throughout the medium and, when a certain proportion of the hydrolytically provided silanols have undergone polycondensation (including cross-linking), the mass becomes stiff enough to qualify as a gel. Of course, other determinant factors regarding the properties of the gels are the nature and the proportion of the dopant metals incorporated in the main or side chains of the present network structure.
Indeed, the distribution (i.e. the average number per silicon atom) of the metal-oxide substituents varies in one or two directions of space according to a predetermined pattern. By this, it is meant that if a defined mass or volume of the gel is considered, the average concentration of the dopant metals in each elemental volume of the gel will vary either, say, in the direction of the width or in the direction of both width and length, but not in the direction of the height. Or, in other words, if the gel is cylindrically shaped the metal concentration can follow a radial gradient pattern, being for instance high in the center and low toward the periphery. Generally speaking, the concentration of the metal dopant in the present gel varies from about 0.01 metal atom to about 0.3 metal atom per silicon atom (1 to 30 at. %). Evidently, if the dopant metal is a divalent or plurivalent metal, it can also participate to the polycondensate backbone of the gel and, therefore, it will then appear as in the following illustrative partial formula where the dopant metal (ME) is a tetravalent metal like germanium. ##STR2##
Of course, if the dopant metal contains further residual valencies (as in formula II above) such will be connected to OR and/or OH groups as for the silicon atoms pictured in formula I.
It has been said that during preparation of the gel, the metal-polysiloxane alkoxide (or compounded polysilicic-polydopant metal acid) cross-linked matrix holds a liquid phase containing alkanols, water, possibly catalysts and uncondensed silicon and other metal alkoxides. This is why the gel is defined as a "wet gel" and is due to the inherent structure of the gel which, as said before, results from the hydrolysis condensation of alkoxysilanes in the presence of variable quantities of other metal alkoxides. Thus, it is easily understandable that the present gel is actually an evolving structure the composition of which may indeed vary depending on the time at which it is considered and the degree of hydrolysis and polycondensation (all other factors being considered constant which is not necessarily so as will be seen later). Hence, the water present is evidently there for hydrolyzing the alkoxide groups and its quantity can vary between wide limits, e.g. between 1 and 30 mole per total moles of silicon and metal alkoxides. Lower alkanols, some part of which being derived from the hydrolysis of the silicon and/or metal alkoxides are also present. Another part of the alkanols is possibly there as a solvent. The alkanols are generally the lower members of the series such as MeOH, EtOH, PropOH and the like. The liquid of the present gel can also contain hydrocompatible solvents such as acetone, ethanolamine and ethyl acetylacetate in quantities ranging from 0.01% to 10% by weight of the gel. The liquid can also contain catalysts, for instance HCl, NH.sub.4 OH. Preferably, the amount of catalyst is not over 10.sup.-2 mole per mole of starting silicon alkoxide.
It should be noted that in some cases, the dopant metals can stay trapped in the gel matrix in a form different from the covalently bonded state previously described. For instance, the metal dopants can be there in the oxide or hydroxide state, being attached to the polycondensed backbone by adsorption (hydrogen bonds, Van der Vaals forces, electron transfer forces or the like). When the gel is eventually converted into glass preforms for the drawing of optical fibers, the distribution of dopant metals in said glass will stay the same as in the original gel.
The conversion of the present gel to a glass can be done according to the usual techniques prevalent in the field of converting alkoxide wet gels to dried liquid free structures followed by densifying and sintering at relatively moderate temperatures (moderate being used relatively to glass melting temperatures). Details concerning such techniques can be found in the following references: YAMANE et al, J. Mat Sc. 13 (1978), 865-7a; S. SAKKA & K. KAMIYA, Proc. Int. Symp. "Sintering of oxide and non-oxide Ceramics Japan (1978), 101-9.
Basically, the original mixture is left to equilibrate at room or higher temperature until the hydrolysis and polycondensation of the silicon and other metal compounds results in the formation of a stiff gel. Ageing of the gel by slowly evaporating the liquid surrounding the gel matrix will prevent crack formation in the matrix. After most of the liquid has been removed, the gel which is arbitrarily defined as a "dry gel" because the point where the solid does not contain any more trapped liquid is very difficult to define, is progressively heated to avoid possible ruptures due to internal pressures of volatiles produced by further condensation reaction and eventual oxidation of carbon containing side-products. Upon further heating, the material can be densified and sintered to a glass. This glass is of very high purity depending on the purity of the starting materials used for making the gel and can be finally used for making optical articles with controlled graded refraction indices, e.g. optical fibers. The geometrical distribution of refractive index follows the same pattern as the dopant metal profile which is the fundamental character of the wet gel made by the invention.